What’s the deal with lights that are supposed to be better for the environment?
In incandescent lamps, an electric current heats up a wire filament until it glows. The filament is typically tungsten, which has a high melting point - you don’t want bits of filament boiling off!
The bulb is filled with an inert gas - there are many choices - but the key is not to have oxygen present; it would react with the filament and cause it to burn. As you can imagine, quite a bit of energy goes into producing heat rather than light (though this would not be a problem in winter, because the heat would help keep your home warm).
Fluorescent lights are much more efficient, with little energy lost to heat. They have been around for a while, usually in the form of long white tubes, but now you can buy them curled up into light-bulb shapes, called compact fluorescent lights, or CFLs.
These have little filaments at the ends, which turn on only briefly at start-up to vaporize some mercury and produce what is basically a long spark along the length of a tube filled with low-pressure mercury vapor.
This spark emits ultraviolet light, which strikes a white phosphor coating inside the glass bulb, which in turn emits visible light.
To produce the same amount of light as an incandescent bulb, a CFL requires one-third or less power.
Another option is light-emitting diode lamps, which use solid-state devices that make red, green, and blue light in a combination intended to look close to white, with very little heat produced.
These alternative lighting choices have some drawbacks, however. They produce light with a spectrum that is quite far from what we’re used to getting from the sun, though “full-spectrum’’ fluorescent lamps are better. This slightly changes our perception of the colors of objects. There is also, for some people, a noticeable flicker with nonincandescent bulbs. And the mercury in fluorescent lights harms the environment if not disposed of carefully.
All in all, there’s a lot to think about besides energy efficiency.
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 
